The life of a new individual is initiated by the fusion of genetic material from the two gametes, the sperm and the egg. After several rounds of division, the cells begin a process of differentiation that ultimately results in the mature adult organism. The process involves many steps including a diverse number of factors which act at specific times during the pathway leading to maturation. The maturation to the adult form does not completely terminate the differentiation process. This is because the adult organism has, in addition to fully differentiated cells, undifferentiated stem cells that are available for both replenishment of differentiated cells during the natural cycle of degeneration and regeneration; and also for repair of damaged tissue. Examples of undifferentiated cells in the adult are bone marrow stem cells (more specifically hematopoeitic stem cells and progenitor cells) as well as endothelial progenitor cells. Cells of this sort provide a therapeutic toolbox in nature for repair and reconstitution of damaged or diseased tissue in a patient. The use of this therapeutic tool box by health care providers for treating patients is limited by the lack of methods to manipulate the differentiation pathways of these cells and to prepare or boost existing numbers of undifferentiated cells without triggering differentiation.
There is a need therefore to find novel methods in which the supply of undifferentiated cells from any particular individual may be increased, for example, by stimulating the proliferation of the cells without inducing differentiation. It is also desirable to modulate differentiation of undifferentiated cells in a controlled manner. Undifferentiated cells that are ready to differentiate when stimulated to do so offer a treatment to subjects that suffer from diseases in which either the stem cells themselves become depleted such as in chemotherapy which destroys bone marrow, or alternatively for diseases in which differentiated cells are being depleted at a rate that is greater than the body can compensate for the loss by means of using the natural supply of undifferentiated stem cells. For example, in AIDS there is a rapid destruction of mature blood cells by the human immune deficiency virus resulting in a dramatic decrease of immune cells in the patient. There is a need to identify factors that cause stem cells to proliferate and that can modulate differentiation so as to enhance the availability of such cells.
The adult organism contains both endothelial stem cells and hematopoietic stem cells (HSC). These cells are undifferentiated but under appropriate conditions, differentiate to form blood cells and blood vessels respectively. Although there have been extensive studies on vascular growth in the adult, it is unknown whether vascular growth is restricted to vessel extension (angiogenesis) or whether there is de novo vascular development (vasculogenesis) also. The understanding of factors that regulate vascular growth is not only important in understanding how to inhibit abnormal vascular growth such as occurs in tumors, rheumatoid arthritis, hemiangiomas, angiofibromas, psoriasis and capillary proliferation and diabetes but also in understanding how to repair vessels after traumatic events including surgery, transplantation and nutrient deprivation to tissues such as occurs in vascular diseases such as cardiovascular or cerebrovascular diseases.
In contrast to vascular growth, hematopoiesis is normally a continuous process throughout the life of an adult. Blood cells are regularly degraded and new cells are formed resulting in a daily production of millions of mature blood cells. Numerous diseases result from imbalances between degradation and reconstitution of blood cells or from generation of inappropriate numbers of certain blood cells. A simplified schematic of blood cell differentiation is provided in FIG. 12. This schematic shows the developmental pathway of eight different types of blood cells that may be derived from a hematopoietic stem cell (HSC) and which passes through an immature progenitor stage. The pluripotent hematopoietic stem cell gives rise to erythrocytes, neutrophils, basophils, eosinophils, platelets, mast cells, monocytes, tissue macrophages, osteoclasts and T and B lymphocytes through a number of different pathways. In the adult, erythrocytes are formed when the pluripotent stem cell differentiates into BFU-E (a burst forming unit-erythroid), which in turn forms a CFU-E (colony forming unit-erythroid). Organs which form blood cells in the adult include bone marrow and to a lesser extent, liver whereas the spleen is the primary site of subsequent clearing of aged or abnormal blood cells. Although the search for factors that regulate hematopoiesis has not been restricted to adults. studies in embryos has been restricted to events that occur when the embryo is already at a relatively advanced stage of development.
With regard to cellular events in the embryo, Cumano et al., Lymphoid Potential, Probed before Circulation in Mouse, Is Restricted to Caudal Intraembryonic Splanchnopleura, 86 (1996) 907-16, proposed that the hematopoietic stem cells (HSC) that populate the adult arise from an intraembryonic site. Blood cells reported to first arise in blood islands in the embryo, appear to originate from hematopoietic progenitor cells in the para-aortic splanchnopleura within the developing embryo. (Cumano et al. (1996). The early development of a mouse is shown in FIG. 14 and the region of early blood island formation is identified on the periphery of the extracoelomic cavity.
At present, there are a number of growth factors that are known to stimulate early stage intermediate cells in different hematopoietic pathways. These include the hematopoletic growth factors, erythropoietin, granulocyte colony stimulating factor (G-CSF) and granulocyte-macrophage colony stimulating factor (GMCSF). For example, CFU-E respond to erythropoietin to produce the first recognizable differentiated member of the erythrocyte lineage, the proerythroblast. As blood oxygen levels fall, erythopoietin levels increase, leading to the production of more red blood cells. As a red blood cell matures, it becomes an erythroblast, synthesizing an enormous amount of hemoglobin and then an erythrocyte. Erythrocytes leave the bone marrow to undertake oxygen delivery to bodily tissues. Although the known factors may have utility in the treatment of certain malignancies or hematologic/immune deficiencies, there is a great need for development of additional therapies, particularly those with a wider range of biological activities that act earlier in the differentiation pathway. The availability of a molecule that could stimulate proliferation and/or differentiation of HSC early in the pathway of differentiation would be especially valuable as a therapeutic. However, there are no factors that are known beyond doubt to stimulate the growth of pluripotent HSC themselves. A protein called stem cell factor has been identified to be associated with pluripotent hematopoietic cells, but this factor is believed to be a survival factor and not a factor capable of stimulating proliferation of these cells (Caceres-Cortes et al., J. Biol. Chem., 269 (1994), 12084-91). There is a need to regulate proliferation and differentiation of hematopoietic stem cells. For example, it would be desirable to inhibit uncontrolled proliferation of stem cells or progenitor cells such as occurs in certain pathological conditions. There is a need for methods to expand the number of pluripotent HSC either in vitro or in vivo for use in treating patients with chronic anemia or those undergoing chemotherapy where the majority of their bone marrow cells are destroyed so that it is necessary to effectively stimulate the remaining cells or for increasing the availability of HSC for transplantation to an anaemic patient.